An extracellular xylanase produced by a cellulase-negative mutant strain of Streptomyces lividans 1326 was purified to homogeneity. The purified enzyme has an apparent Mr of 43,000 and pI of 5.2. The pH and temperature optima for the activity were 6.0 and 60 degrees C respectively, and the Km and Vmax. values, determined with a soluble oat spelts xylan, were 0.78 mg/ml and 0.85 mmol/min per mg of enzyme. The xylanase showed no activity towards CM-cellulose and p-nitrophenyl beta-D-xyloside. The enzyme degraded xylan, producing mainly xylobiose, a mixture of xylo-oligosaccharides and a small amount of xylose as end products. Its pattern of action on beta-1,4-D-xylan indicates that it is a beta-1,4-endoxylanase (EC 3.2.1.8).

The potential of Pseudomonas pseudoflava to produce poly-,1-hydroxyalkanoates (PHAs) from pentoses was studied. This organism was able to use a hydrolysate from the hemiceliulosic fraction of poplar wood as a carbon and energy source for its growth. However, in batch cultures, growth was inhibited completely at hydrolysate concentrations higher than 30% (vol/vol). When P. pseudoflava was grown on the major sugars present in hemicelluloses in batch cultures, poly-4-hydroxybutyric acid (PHB) accumulated when glucose, xylose, or arabinose was the sole carbon source, with the final PHB content varying from 17% (wtlwt) of the biomass dry weight on arabinose to 22% (wt/wt) of the biomass dry weight on glucose and xylose. Specific growth rates were 0.58 h-1 on glucose, 0.13 h-1 on xylose, and 0.10 h-1 on arabinose, while the specific PHB production rates based on total biomass ranged from 0.02 g g-1 h-1 on arabinose to 0.11 g g-1 h-1 on glucose. PHB weight-average molecular weights were 640,000 on arabinose and 1,100,000 on glucose and xylose. The absolute amount of PHB in the cells decreased markedly when nitrogen limitation was relaxed by feeding ammonium sulfate at the end of the PHB accumulation stage of the arabinose and xylose fermentations. Copolymers of ,-hydroxybutyric and ,-hydroxyvaleric acids were produced when propionic acid was added to shake flasks containing 10 g of glucose liter-'. The ,-hydroxyvaleric acid monomer content attained a maximum of 45 mol% when the initial propionic acid concentration was 2 g liter-'.

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